Patent Document 1 discloses a known example of a tire position determination system (auto-location function) that automatically determines tire positions to monitor the air pressure of each tire. The system of Patent Document 1 includes first sensors (4a to 4d), which are respectively arranged in wheels (2a to 2d), four second sensors (5a to 5d), which correspond to specific positions of a vehicle, and a measurement system (3), which determines wheel positions. The first sensors transmit signals (S4a to S4d) that indicate wheel positions to the measurement system. The second sensors measure angular positions of wheels and calculate measurement values (S5a to S5d) of the wheel angle positions. The measurement system determines the wheel positions by obtaining phase positions (W1a to W3a and W1b to W3b) of the signals of the first sensors from the measurement values and checking whether or not the phase positions remain within predetermined allowable ranges (WTa and WTb) during a predetermined monitoring period.